1. Field of the Invention
The present invention relates to a system capable of low-latency image discrimination. More particularly, the present invention relates to an apparatus and method by which both spectral analysis and complex polarization analysis can be performed at high rates and over a wide range of wavelengths regardless of polarization state of the image data.
2. Description of the Invention Background
Among the obstacles that must be overcome in the development of automated machine vision systems is the ability to collect and process spectral data at varying wavelengths and at high rates of speed and to recognize and distinguish among various objects and their background. However, in all but the most simple of scenarios, such task-oriented vision apparatus suffer from low speed, high cost, great complexity and limited object identification and recognition capabilities.
In such vision systems, shape is the most commonly used object identifier. The greatest utility in improving the capabilities of such object identification systems has been shown by systems that utilize the spectral and polarimetric signatures of objects to identify and discriminate the object from its background. Spectral imaging systems typically decompose scenes into their component colors and wavelengths and discriminate the various objects in the scene according to their spectral composition. One means of accomplishing the spectral discrimination function is by use of an acousto-optic tunable spectral filter (AOTF). An AOTF is an electronically tunable optical bandpass filter. AOTF's typically consist of a specially selected birefringent crystal equipped with a means of generating an RF acoustic drive signal through the crystal. An incident beam of radiation having a first polarization state, is directed into the crystal and an acoustic wave is propagated nearly perpendicular to the incident beam within the crystal. The incident beam is diffracted by the RF drive signal and is shifted from the first polarization state to a second polarization state over a selected passband of optical frequencies, as determined by the frequency of the acoustic wave. Thus, only the portion of the incident radiation with a wavelength within the passband (or acceptance wavelength) experiences a shift in polarization. The acceptance wavelength of the filter is electronically tunable by varying the frequency of the RF drive signal within the crystal. As such, the AOTF operates to pass (or accept) that portion of the beam having a wavelength within the band determined by the RF drive signal within the crystal and to block other portions of the beam. By electronically tuning the passband of the AOTF such that the acceptance wavelength varies across a discrete portion of the spectrum, only objects in the subject scene whose wavelengths fall within that discrete portion of the spectrum will be allowed to pass through the AOTF and will be visible.
The acceptance band of solid state tunable AOTFs can be electronically varied at rapid rates by tuning the frequency of the RF drive signal across a given range of frequencies. If some portion of the light in the incident beam is within the acceptance band of wavelengths it is shifted from the first polarization state to the second polarization state, and those objects in the scene that contain wavelengths within this band are diffracted while all other objects are not. As such, the objects that contain wavelengths within this band exhibit a flashing sensation, while the others remain constant. Thus, the AOTF can be used to scan a scene across a series of wavelengths and thereby identify and differentiate objects of specific spectral signatures from other objects and background clutter having different spectral signatures. The responsiveness provided by the solid-state operation of existing AOTFs enables this discrimination process to be performed at rapid rates approaching and exceeding real-time video rates.
This ability to distinguish the various objects in the field of view from each other lends itself to use in applications where objects may be visible but difficult to detect with the naked eye because they are indistinguishable from the background against which they are placed. Such applications include target recognition and threat detection and acquisition systems, in which the target or threat may be camouflaged by the surrounding environment, and autonomous vehicles that must be capable of discriminating among objects that lie in their path and determining whether or not they need be avoided.
Objects of interest in these and other scenarios can be discriminated with enhanced sensitivity if filtering is performed not only for spectral composition, but also for polarization state of the objects in the scene. For example, depending on chemical composition or surface structure, certain objects in a scene preferentially reflect radiation at linearly polarized states at certain wavelengths while other objects in the scene may reflect (or in the infrared, may emit) radiation that is circularly or elliptically polarized at certain wavelengths. For example, man-made objects (i.e., vehicles, structures, weaponry, etc.) generally differ from natural objects (i.e., vegetation, soil, rocks, etc.) in that they preferentially reflect radiation with a given linear polarized state at different wavelengths than do natural objects. Thus, by tuning the acceptance wavelength of the AOTF to that portion of the spectrum typically occupied by man-made objects, it is possible to discriminate man-made objects from their background. Even where shape or color alone may not suffice to extract an object from its background, the data from the AOTF will supply an additional discriminant to use in distinguishing an object from its surrounding environment.
As mentioned above, while some objects may reflect or emit radiation that is linearly polarized at certain wavelengths, other objects in the scene may reflect or emit radiation that is circularly or elliptically polarized at certain wavelengths. Object discrimination can be enhanced by analyzing the polarization characteristics of radiation having wavelengths that exhibit complex circular or elliptical polarization states as well as linear polarization states. However, a circularly or elliptically polarized incident beam cannot be made subject to detection by intensity variation using an AOTF. One means of enabling an AOTF to subject circularly or elliptically polarized incident beams to intensity variation for visualization and object discrimination is to first pass the beam through a phase retarder. Phase retarders have the property that they convert elliptically (or circularly) polarized light to linearly polarized light and therefore permit the AOTF to subject the incident light to the intensity analysis described above.
A typical phase retarder will include a phase plate comprised of a birefringent crystal, such as calcite, with ordinary and extra-ordinary refractive indices n.sub.o and n.sub.e. The crystal is accurately polished to a thickness L, so that when radiation at a specific wavelength .lambda. passes through the crystal, the ordinary and extra-ordinary radiation will experience a relative phase difference or modulation of .pi./2 radians. As presented, the phase modulation relationship .DELTA..phi. governing this process is: EQU .DELTA..phi.=2.pi.L(n.sub.e -n.sub.o)/.lambda.=.pi./2 (1)
As dictated by the fact that the thickness L and refractive indices n.sub.o and n.sub.e are fixed, a given retardation plate is suited for use only with incident radiation of a specific wavelength. Thus, each particular wavelength to be analyzed requires a retardation plate specifically constructed for that particular wavelength. It is axiomatic then, that a complete analysis of a typical scene containing incident light of numerous wavelengths would require an equally numerous set of retardation plates. In operation, each plate would have to be sequentially positioned in the spectral imaging system such that each wavelength encountered can be individually addressed. Such a system would prove too cumbersome and expensive to be viable. The rate at which data could be gathered would be severely limited and unacceptably slow for the aforementioned applications.
A need thus exists for robust apparatus and methods providing for low-latency analysis of both spectral and complex polarization signatures, at high rates of recognition, in the presence of complex and varying background polarization signatures.